1. Field of the Invention
The present invention relates to a light emitting device that includes a light emitting element containing a luminous body in a single layer or a laminate of plural layers between a pair of electrodes, and a method of manufacturing the same. In particular, the present invention relates to a technique in which a short-circuit or leak area developing in a step of manufacturing the light emitting element can be repaired with a simplified method.
2. Description of Related Art
As an example of a light emitting element formed using a light emitting medium called an organic electroluminescent material, there is a structure in which an organic amine-based hole transport layer and a layer containing an organic compound such as a tris-(8-quinolinolato) aluminum complex (Alq3) that exhibits an electron conduction property and also has a luminescent property, are laminated between a pair of electrodes. The light emitting element can obtain a luminance of several hundred cd/cm2 through application of DC voltage at 6 to 8V.
In the light emitting element, when layers responsible for emitting light directly or indirectly are functionally represented, distinction may be made among layers, like a light emitting layer, a hole injection layer, an electron injection layer, a hole transport layer, an electron transport layer, and the like. These functional representations occasionally enable distinguishment clearly on the layer basis, but cannot lead to clear distinguishment on the layer basis because a layer formed as a part of a mixed structure in some cases. An extremely simple structure has an anode/a light emitting layer/and a cathode, which are laminated in this order. In addition to this structure, there are other structures having an anode/a hole injection layer/a light emitting layer/and a cathode, and having an anode/a hole injection layer/a light emitting layer/an electron transport layer/and a cathode, laminated in the stated orders.
A light emitting element operating normally shows a rectification property in which a so-called current-voltage characteristic as a diode has is observed. That is, when applying a forward bias to the light emitting element, a current increases exponentially in response to the applied voltage. When applying a reverse bias, a current hardly flows until the voltage reaches a breakdown voltage. In order to cause the element to emit light, charge is necessarily injected, and therefore a forward bias is applied.
JP 8-234683 A discloses an example of a light emitting device having an active matrix driving system that controls the above-mentioned light emitting element with a field effect transistor. According to JP 8-234683 A, a structure is disclosed in which an organic electroluminescent layer is formed as an upper layer on a thin film transistor (TFT) made from polycrystalline silicon via an insulating film formed of silicon dioxide. Further, a passivation layer having an end processed into a tapered shape on an anode, is positioned on a lower layer side of the organic electroluminescent layer. Furthermore, a material having a work function of less than 4 eV is selected to form a cathode. An alloy of metal such as Ag or Al and Mg may be adapted.
Incidentally, when a reverse voltage, which is not involved in emitting light, is applied to the light emitting element, it is empirically known that a life of the element extends. JP 2001-109432 A discloses a light emitting device of an active matrix driving system that utilizes this phenomenon and applies a reverse voltage in a non-light emitting period according to synchronous timings of input video image data.
Meanwhile, as to a solar cell or the like that forms diodes with semiconductor thin films, various techniques are experimented for repairing a short-circuit portion by applying a reverse voltage. An example of the technique is disclosed in U.S. Pat. No. 6,365,825 B in which a current concentratedly flows on a short-circuit portion by the reverse voltage application, and a short-circuit defect is repaired by insulating the portion with use of heat generated due to Joule heat.
FIG. 9(A) schematically shows a light emitting element that includes short-circuit defects caused by a pinhole 14 and a foreign matter 15 mixing therein, and is a drawing useful in explaining the reverse voltage effect. When there is a short-circuit defective portion 14 in a diode element 10 including a thin film 12 that enables a rectifying contact or a rectifying junction between a pair of electrodes composed of a anode 11 and a cathode 13, a current equal to or higher than a reverse saturation current flows via the portion upon application of a reverse voltage.
As schematically shown in FIG. 9(B) using points A and B indicated by the dotted lines at the time of applying the reverse voltage, a current-voltage characteristic of the diode element 10, the reverse current rapidly increases at a certain voltage. For example, at the short-circuit defect caused by the short-circuit defective portion 14 including the pinhole, the reverse current flows at a relatively low voltage because a cathode material wraps around the portion. Further, when the minute foreign matter 15 is included in the element, a withstand voltage becomes lower, forming the short-circuit defective portion 15 where the reverse current increases at a breakdown voltage or lower due to dielectric breakdown.
At this time, the current concentratedly flows on the short-circuit defective portions 14 and 15, and a current density increases. Heat is thus generated to reach a high temperature, whereby the portion is modified to be insulated. Therefore, from the second voltage scanning operation onward, a normal diode characteristic can be obtained. Even if the short-circuit defective portion is not repaired by scanning a voltage once, when the voltage scanning is repeated plural times, a probability of repairing the defective portion increases. In this way, the short-circuit portion is insulated to be repaired by the application of a predetermined reverse voltage.
The repairing of the short-circuit portion through the application of a reverse voltage can be conducted relatively easily. However, a principle of the repairing is to utilize a heat generation phenomenon caused by the current concentration. It is thus necessary to make a large current flow instantly. Therefore, a constant voltage source having a current supply capacity suitable for the flow of large current is required for a power source to be applied.
However, as to a drain current of a TFT used in an active matrix driving system, when a gate voltage is determined as shown in FIG. 10, a flowing current almost saturates no matter how the drain voltage is increased. That is, as long as the operation is executed in a saturation region of the TFT, this is equivalent to a case where the connection is made to the constant current source. Also, even when the operation is executed in a linear region, this leads to the same result, in other words, a current equal to or higher than the saturation current is not allowed to flow. Even when a reverse voltage is applied via the TFT, a limitation is placed on a maximum current value. Therefore, such a short-circuit defect shown in FIG. 9 cannot be insulated sufficiently.
The present invention has been made with a view toward solving the above-mentioned problems, and it is therefore an object to stabilize a luminance and prevent a degradation during use in a display device of the active matrix driving system that arranges TFTs in matrix, by completely repairing the defective portion through the application of a reverse voltage and repairing the short-circuit or leak area of the light emitting element.